1. Field of the Invention
The disclosures herein generally relate to a step-up switching regulator controlling output voltage with a PWM signal, especially, a switching regulator which prevents inrush current from being generated while starting the output voltage.
2. Description of the Related Art
Conventional switching regulators are described, for example, in patent documents 1-5.
FIG. 7 is a block diagram illustrating a configuration of a switching regulator taken as a first example of conventional art. The switching regulator shown in FIG. 7 is a step-up switching regulator adjusting brightness of white LEDs (WLEDs) in response to a PWM signal input as an enable signal EN. FIG. 8 is a timing chart explaining behavior of the switching regulator shown in FIG. 7.
The switching regulator shown in FIG. 7 generates an output voltage Vout from power supply voltage Vcc, which is applied to n white LEDs (WLED1-WLEDn), which are all the same type and connected in series. An inductor L1 and a switching transistor MN1 are connected in series between the power supply voltage Vcc and ground potential GND, to which input current Iin is supplied from the power supply voltage Vcc. A node between the inductor L1 and the switching transistor MN1 is connected to a capacitor C1 and WLED1-WLEDn via a rectifier D1. WLED1-WLEDn are connected to ground via a resistor R1. A reference voltage circuit 1 generates a reference voltage Vref, which is sent to a PWM signal converting circuit 2. The PWM signal converting circuit 2 receives as input the PWM signal as the enable signal EN, changes the reference voltage Vref in response to a duty cycle of the enable signal EN (the PWM signal), which is sent as input to a non-inverted input terminal of an error amplifier circuit 3. An inverted input terminal of the error amplifier circuit 3 receives as input a feedback voltage Vfb generated between WLED1-WLEDn and the resistor R1. The error amplifier circuit 3 generates an error difference voltage Verr indicating an error difference between the reference voltage Vref and the feedback voltage Vfb, which is sent to a switching control circuit 4. The switching control circuit 4 further receives as input a slope voltage Vslp and a clock signal CLK generated by an oscillator 6, and a limiting signal Vlim corresponding to a predetermined current limit value Isw generated by a current limiting circuit 7. The switching control circuit 4 generates a switching signal PWMOUT based on the error difference voltage Verr, the slope voltage Vslp, the clock signal CLK, and the limiting signal Vlim. Specifically, the switching control circuit 4 compares the error difference voltage Verr and the slope voltage Vslp to generate the switching signal PWMOUT, which controls the duty cycle of the switching transistor MN1 to make the feedback voltage Vfb become equal to the reference voltage Vref (PWM control). The switching control circuit 4 applies the switching signal PWMOUT to the gate of the switching transistor MN1 via an output buffer circuit 5.
In the switching regulator shown in FIG. 7, the output voltage Vout is the sum of the feedback voltage Vfb and forward direction voltage of a white LED multiplied by n, where n is the number of LEDs, WLED1-WLEDn, connected in series. A current flowing through WLED1-WLEDn, Iwled, is given by Iwled=Vfb/R1. Since the reference voltage Vref varies in response to the duty cycle of the enable signal EN (PWM signal), it is possible to change arbitrarily the value of the feedback voltage Vfb with the duty cycle of the enable signal EN, which in turn makes it possible to change arbitrarily the value of the current Iwled through WLED1-WLEDn, and thus, the brightness of WLED1-WLEDn can be adjusted.
In the switching regulator shown in FIG. 7, as shown in FIG. 8, when starting the output voltage Vout, the feedback voltage Vfb is equal to ground until the output voltage Vout becomes greater than the voltage of the forward direction voltage of the white LED multiplied by n, where n is the number of LEDs, WLED1-WLEDn, connected in series. As a result, the error difference voltage Verr takes a high level value, which prevents feedback control of the switching regulator from working, and the duty cycle gets close to the maximum duty cycle. At this moment, for example, an input current Iin reaches the current limit value Isw set by the current limiting circuit 7, which means that an inrush current is generated. This is a problem in that a device at the preceding stage of the switching regulator gets influenced by the inrush current.
FIG. 9 is a block diagram illustrating a configuration of a switching regulator taken as a second example of conventional art. FIG. 10 is a timing chart explaining behavior of the switching regulator shown in FIG. 9.
To solve the problem of inrush current with the switching regulator shown in FIG. 7, the switching regulator shown in FIG. 9 further provides a soft-start circuit 8 to generate a soft-start voltage Vss, which is a reference voltage used for the soft start. Also, instead of the switching control circuit 4 in FIG. 7, the switching regulator shown in FIG. 9 provides a switching control circuit 9 operating in response to the soft-start voltage Vss received as input. As shown in FIG. 10, the soft-start voltage Vss increases gradually when the enable signal EN takes a high level value. The switching control circuit 9 shown in FIG. 9 compares the slope voltage Vslp and the lower one of the error difference voltage Verr and the soft-start voltage Vss, and generates the switching signal PWMOUT having the duty cycle according to the comparison result.
As a conventional switching regulator using soft-start voltage, for example, the patent document 1 discloses an invention. Also, as an improved usage of soft-start voltage, a switching regulator having a clamp circuit to set an upper limit of error difference voltage is disclosed, for example, in the patent document 3.
With conventional switching regulators explained above, by increasing the duty cycle of the switching transistor MN1 gradually after the enable signal EN takes a high level value, it is possible to control the input current Iin and the current Iwled through the WLED1-WLEDn while starting the output voltage Vout. However, since the duty cycle is determined with the comparison between the slope voltage Vslp and the lower one of the error difference voltage Verr and the soft-start voltage Vss, in a transition state in which the error difference voltage Verr starts to take a lower value than the soft-start voltage Vss, the input current Iin becomes greater than a steady state current, which means that an inrush current is generated. Also, at the moment when the error difference voltage Verr starts to decrease, the output voltage Vout has completely started, the feedback voltage Vfb has become equal to the reference voltage Vref, and the input current Iin is close to the steady state current. Therefore, if an inrush current is generated at this moment, a problem arises in that an overshoot from the steady state becomes too large.